Invalid boundary region to join faces
Having a hole in an NGon is not a good idea. faces have to be filled. An NGon is made of invisible triangles but the link between vertices has to exist! Leaving a few edges makes is possible without crossing the hole.

Your face is smooth (flat would be more exact), unless you want to subdivide it, what you'd better not do with an NGon. Having more edges is not a problem, it's a solution! it won't cause any problem printing it. (it still needs thickness)

NGons are just supposed to make things easier and clearer, some sort of shortcuts, but knowing what is topology and why you sometimes need more is crucial for doing things that looks good.